The present invention relates generally to transducers, and more particularly to orthomode transducers and their method of manufacture.
As known in the art of front-end electronics, the orthomode transducer (OMT) is a three-port device which can be used to separate and/or combine orthogonally polarized signals. In a conventional use, the OMT is used to receive signals of a first polarization, and transmit signals of an orthogonal polarization using the same antenna.
FIG. 1 illustrates a perspective view of a conventional Ku-band OMT 100 known in the art. As shown, the OMT includes a transmit section 110, a receive section 130, and an antenna section 150. The transmit section 110 is configured to transmit a vertically-polarized signal 125 (typically in the range from 14-14.5 GHz for Ku band operation) and includes a transmit port 112 containing a substantially rectangular aperture 114. The transmit section 110 may further include an internal septum 116 for improving the impedance match between the transmitter circuitry and the antenna (neither shown).
The receive section 130 is configured to receive a horizontally-polarized signal 135 (typically in the range from 10.95-12.75 GHz for Ku band operation) and includes a receive port 132 containing a substantially rectangular aperture 134. The antenna section 150 includes an antenna port 152 containing a circular aperture 154 which is sized to support the transmission of the outgoing vertically-polarized signal 125, or the reception of the incoming horizontally-polarized signal 135, thereby operating over the combined transmit/receive band of 10.95-14.5 GHz. The wideband OMT permits the use of a single antenna for transmission and reception, which greatly reduces the system's cost.
Unfortunately, wideband OMTs, such as the exemplary 10.95-14.5 GHz OMT, face a common obstacle in that the dimensions of the antenna section 150 usually permit the propagation of higher, deleterious waveguide modes. In particular, if the cutoff frequency for the antenna section 150 is designed at 10 GHz, a higher order TM01 waveguide mode will exist at approximately 13.05 GHz, occurring in the upper band of the OMT's operation. The existence of this mode means that signals within the antenna section 150 above 13.05 GHz may propagate in either the dominant mode TE11, in the higher mode TM01, or possibly both. Signal propagation in the dominant TE11 mode is desired as it produces the intended vertically-polarized signal. Signal propagation in the TM01 mode is undesirable, as it will propagate as a cross-polarized signal, e.g., a horizontally-polarized signal in the present example. As a result of this, the intended vertically-polarized signal will have reduced signal strength. Moreover, the signal may experience mode conversion in which it switches between the TE11 and TM01 signal modes, resulting in significant signal distortion.
FIG. 2 illustrates a H-plane response of the conventional Ku-band OMT shown in FIG. 1. Trace 210 shows the amplitude of the desired co-polarization signal, and trace 230 shows the undesired cross-polarization signal. The amplitude of the transmitted cross-polarization signal 230 rises to 23 dB relative to the co-polarization signal 210.
The presence of the undesired cross-polarization signal 230 indicates the corresponding presence of TM01 mode signals propagating in the antenna section 150 which, as explained above, depletes power from the desired transmitted signal and contributes to significant signal distortion. What is therefore needed is an OMT having improved cross-polarization suppression.